In many MEMS device (e.g., MEMS gyroscopes, accelerometers, resonators, switches, and other types of devices), movement of a mass may be driven, adjusted, and/or sensed using one or more electrodes placed at least partially adjacent to the mass. Such interaction between the electrode and the mass may be electrostatic, although other types of configurations may be used (e.g., piezoelectric). For example, as shown schematically in FIG. 1, an electrode 604 may be placed adjacent to a mass 602 in substantially the same plane as the mass 602. FIG. 1A shows a side view and FIG. 1B shows a top view of an exemplary mass/electrode system. Typically, the electrode 604 is formed on or otherwise anchored to an underlying substrate or support structure (not shown). The electrode 604 also may be attached or otherwise anchored laterally to a side structure. The electrode 604 may be constructed in whole or in part from the same material as the mass 602 (e.g., the mass 602 and electrode 604 may be formed from the top silicon layer of a silicon-on-insulator wafer) and/or may include other materials/layers (e.g., formed by deposition of one or more materials on the silicon wafer).
Ideally, the electrode 604 is perfectly stationary, with a precise gap between the electrode 604 and the stationary mass 602. In practice, however, any of a number of factors can cause the electrode 604 to move, even slightly, and such movements can introduce errors into the system. For example, the electrode 604 may deflect due to movement of the device (e.g., an acceleration) and/or electrostatic interaction of the electrode 604 with the mass 602. Among other things, the electrode may move or pivot out-of-plane as depicted by arrows 605 and 606, may move toward or away from the mass 603 in-plane as depicted by arrows 607, may pivot in-plane as depicted by arrows 608 and 609, and/or may translate sideways within the plane as depicted by arrows 612.
Of course, other mass/electrode configurations are often used in MEMS devices, such as electrodes placed entirely or partially above or below the mass, or electrode “fingers” interdigitated with corresponding structures on the mass. Furthermore, other types of electrodes are often used in MEMS devices, such as piezoelectrically coupled electrodes. Such electrodes are similarly subject to movements that can cause erroneous behavior, such as increased sensitivity to external forces (i.e., unwanted forces such as unwanted acceleration), erroneous signals, and reduced performance in MEMS devices such as gyroscopes, accelerometers, and other types of MEMS devices.
U.S. Pat. No. 7,134,340, which is hereby incorporated herein by reference in its entirety, discloses elongated finger structures (e.g., drive and/or sense electrodes) including elongated or multiple anchors to mitigate certain types of electrode movements, particularly pivoting/twisting movements in-plane about the anchor point.
Bulk acoustic wave (“BAW”) gyroscope use has increased in recent years. This trend is driven by their many benefits including, among other things, their high gain factor, which causes them to use less power than conventional gyroscopes. In addition, such gyroscopes generally cost less to manufacture.
To those ends, many bulk acoustic wave gyroscopes known to the inventors have a proof mass (with any polygon shape, e.g., circular or rectangular) with a crystal lattice that, during either or both an actuation or detection phase, vibrates/resonates at a very high frequency, typically in the megahertz range. This is in contrast to gyroscopes having a mass mechanically moving back and forth about a substrate in both phases. When the crystal lattice of the mass vibrates, the mass is considered to be operating in a “bulk” mode.
Some exemplary BAW gyroscope configurations are discussed in Johari, H., Micromachined Capacitive Silicon Bulk Acoustic Wave Gyroscopes, Georgia Institute of Technology, December 2008 and in the following U.S. patents and published patent applications: U.S. Pat. No. 7,895,892, U.S. Pat. No. 7,874,209, U.S. Pat. No. 7,543,496, U.S. Pat. No. 7,427,819, US 2009/0266162, US 2008/0180890, US 2008/0054759, US 2007/0284971, and US 2006/0238078, each of which is hereby incorporated herein by reference in its entirety.
In addition to being subject to external forces, the drive and sense electrodes in shell-type MEMS gyroscopes (e.g., flexure mode and BAW mode gyroscopes) may be subject to very high forces due in part to the high frequencies of operation of such devices, and such forces can deflect the electrodes which in turn can distort the angular rate sensitivity of the gyroscope, causing errors in the system.